The present invention relates to a process for preparing nacreous pigments which have improved gloss properties and are based on mica flakes coated with metal oxides, e.g., mica coated with a layer of hydrated titanium dioxide in aqueous suspension and then washed, dried and heated.
The intensity and purity of the interference color of these pigments depends decisively on the formation of the metal oxide layer precipitated on the mica flakes. In addition to a homogeneous layer thickness, the chief requirements are that the layer is formed free of cracks and displays as little light scattering as possible. It is known that both cracks, which especially may occur in thicker layers when the pigments are heated, and, in particular, the size of the metal oxide crystals precipitated on the mica, play a decisive role in determining the degree of scattering. It is also known that the scatter in the layer in the area in question is greater the larger the individual crystals of the precipitated metal oxide.
The diffuse light formed at the scatter centers in the metal oxide layer on the one hand reduces the intensity at the glancing angle of the reflected portion of light and hence the color saturation, and on the other hand causes a whitening of the interference color. If the crystals are too large, the nacreous luster and the interference color disappear completely.
According to earlier Federal Republic of Germany Application P 31 51 354, corresponding to U.S. application No. 453,045 of Dec. 27, 1982, it had been found that very homogeneous and stable precipitates which do not tend to form cracks when heated are achieved by simultaneous precipitation of SiO.sub.2 and Al.sub.2 O.sub.3 together with the hydrated titanium dioxide. These pigments have an increased transparency and tinctorial strength and can very advantageously be used for all the customary purposes. However, because SiO.sub.2 and Al.sub.2 O.sub.3, which both have a low refractive index, are also precipitated, the refractive index of the metal oxide layer is significantly reduced in comparison with a pure TiO.sub.2 precipitate. This means that the proportion of light reflected at the surface of these pigment particles is less than that in the case of pure TiO.sub.2 /mica pigments.
Although the velvety gloss achieved with these earlier pigments is particularly suitable for many applications, pigments with sharp gloss and bright sparkle are required for other applications.